The invention relates to an apparatus for enabling and/or activating/deactivating a security device, in particular an access device for a motor vehicle, in which, after receiving a stimulation sequence which is transmitted by a transmitter of a transmission device, coded information (enable signal) is transmitted by a portable transmission unit (radio key) and is received by a receiver in the transmission device, with a controller comparing the received enable signal with predetermined coded information and, if they match, emitting an actuation signal to the security device.
In order to improve the convenience of use of access security devices, for example the central locking system of motor vehicles, electronic radio keys have recently come into use, which transmit an electromagnetic enable signal to the security device on pressing a button or completely automatically, and the said device then enables access. In these devices, the radio keys replace the conventional mechanical keys.
An apparatus for operating a door locking and/or alarm system is known from WO 92/18732. In this system, the user carries a radio key in which an enable code is stored in order to activate and deactivate the said security devices. The motor vehicle contains a transmission apparatus, which receives an enable signal transmitted by the radio key, and compares it with a stored coding. If the codes match, an actuation signal is transmitted to the security device (central locking system, alarm system).
The enable signal is transmitted only when the person carrying the radio key is at a predetermined distance from the motor vehicle, which is within the range of the transmitter in the transmission apparatus. The radio key is activated by a stimulation sequence from the security device, without any manual action by the user.
Such a system is generally supplied with a number of matching radio keys. Normally, these keys may be distributed between a number of vehicle users. It is thus possible, once the stimulation sequence has been transmitted, for the security device to receive a response signal from a number of radio keys within range of the security device. This can lead to a collision resulting from all the keys transmitting signals without any coordination, so that the controller of the security device is unable to correctly identify an enable signal and to decide which of the signals is intended to be used as an enable signal.
Systems are known which, if there is any such suspicion, address all the radio keys in a system globally and, if the response is not clear, address individual radio keys selectively, one after the other. This has the disadvantage that, as the number of radio keys which a security device may have and which are located within range of the security device increases, the time required for the data interchange increases, so that the vehicle user is not allowed access to the vehicle until a noticeable time period has passed.
Since, as a rule, the radio key signals are quite weak and are transmitted poorly over a lengthy distance, interference can occur if the transmission path is poor. If interference occurs, the security device must once again select a radio key, and request it to transmit an enable signal. This once again results in a noticeable delay.